This explains how it is important to recycle used water into clean resources. The process starts with Preliminary Treatment where big debris that may include rags and grit are filtered away. Then, in Primary Treatment, large clarifiers allow heavy organic solids to settle down forming sludge. The Secondary (Biological) Treatment is the most important stage whereby desirable bacteria are added in aeration tanks to decompose and digest dissolved organic matter. This can be succeeded by Tertiary Treatment (Polishing) to eliminate such nutrients as phosphorus and nitrogen.